Assessing Oxidative Stress in Tumors by Measuring the Rate of Hyperpolarized [1-13^{13}C] Dehydroascorbic Acid Reduction Using 13^{13}C Magnetic Resonance Spectroscopy

Rapid cancer cell proliferation promotes the production of reducing equivalents, which counteract the effects of relatively high levels of reactive oxygen species (ROS). ROS levels increase in response to chemotherapy and cell death while an increase in antioxidant capacity can confer resistance to chemotherapy and is associated with an aggressive tumor phenotype. The pentose phosphate pathway (PPP) is a major site of NADPH production in the cell, which is used to maintain the main intracellular antioxidant, glutathione, in its reduced state. Previous studies have shown that the rate of hyperpolarized [1-$^{13}$C]dehydroascorbic acid (DHA) reduction, which can be measured $\textit{in vivo}$ using non-invasive $^{13}$C magnetic resonance spectroscopic imaging, is increased in tumors and that this is correlated with the levels of reduced glutathione. We show here that the rate of hyperpolarized [1-$^{13}$C]DHA reduction is increased in tumors that have been oxidatively pre-stressed by depleting the glutathione pool by buthionine sulfoximine treatment. This increase was associated with a corresponding increase in PPP flux, assessed using $^{13}$C-labeled glucose, and an increase in glutaredoxin activity, which catalyzes the glutathione-dependent reduction of DHA. These results show that the rate of DHA reduction does not depend only on the level of reduced glutathione, but also on the rate of NADPH production, contradicting the conclusions of some previous studies. Hyperpolarized [1-$^{13}$C]DHA can be used therefore to assess the capacity of tumor cells to resist oxidative stress in vivo. However, DHA administration resulted in transient respiratory arrest and cardiac depression, which may prevent translation to the clinic.Work in K.M. Brindle’s laboratory is supported by a Cancer Research UK Programme grant (17242) and the CRUK-EPSRC Imaging Centre in Cambridge and Manchester (16465). K.N. Timm was in receipt of MRC and Cancer Research UK studentships, B.W.C. Kennedy and P. Dzien Cancer Research UK studentships and F. Bulat a CRUK -EPSRC Imaging Centre imaging center studentship. I. Marco -Rius acknowledges the European Union Seventh Framework Programme (FP7/2007-2013) for support under the M arie Curie Initial Training Network METAFLUX (project number 264780)

Imaging tumor metabolism to assess disease progression and treatment response

Changes in tumor metabolism may accompany disease progression and can occur following treatment, often before there are changes in tumor size. We focus here on imaging methods that can be used to image various aspects of tumor metabolism, with an emphasis on methods that can be used for tumor grading, assessing disease progression, and monitoring treatment response

Assessing oxidative stress in tumors by measuring the rate of hyperpolarized [1-13C] dehydroascorbic acid reduction using 13C magnetic resonance spectroscopy

Rapid cancer cell proliferation promotes the production of reducing equivalents, which counteract the effects of relatively high levels of reactive oxygen species. Reactive oxygen species levels increase in response to chemotherapy and cell death, whereas an increase in antioxidant capacity can confer resistance to chemotherapy and is associated with an aggressive tumor phenotype. The pentose phosphate pathway is a major site of NADPH production in the cell, which is used to maintain the main intracellular antioxidant, glutathione, in its reduced state. Previous studies have shown that the rate of hyperpolarized [1-13C]dehydroascorbic acid (DHA) reduction, which can be measured in vivo using non-invasive 13C magnetic resonance spectroscopic imaging, is increased in tumors and that this is correlated with the levels of reduced glutathione. We show here that the rate of hyperpolarized [1-13C]DHA reduction is increased in tumors that have been oxidatively prestressed by depleting the glutathione pool by buthionine sulfoximine treatment. This increase was associated with a corresponding increase in pentose phosphate pathway flux, assessed using 13C-labeled glucose, and an increase in glutaredoxin activity, which catalyzes the glutathione-dependent reduction of DHA. These results show that the rate of DHA reduction depends not only on the level of reduced glutathione, but also on the rate of NADPH production, contradicting the conclusions of some previous studies. Hyperpolarized [1-13C]DHA can be used, therefore, to assess the capacity of tumor cells to resist oxidative stress in vivo. However, DHA administration resulted in transient respiratory arrest and cardiac depression, which may prevent translation to the clinic